This invention relates to an apparatus for molding a hollow molded article of a thermoplastic synthetic resin, and more specifically to an apparatus for producing a hollow molded article by a split mold from a heat-softened plastic molding material in the form of a tube or two parallel-laid sheets (such a molding material is generically referred to herein as a parison).
The invention also pertains to an apparatus which can be effectively used in the production of deep-drawn molded articles having raised and depressed surface portions of large height or depth, or hollow molded articles of a complex shape in which the hollow portion is divided into a plurality of sections. Such a hollow molded article of a complex shape is, for example, a collector of a solar heater.
A typical method for molding a hollow article from a parison using a split mold is blow molding. Blow molding, in theory, is a molding method whereby a bag-like molding material is expanded in a closed split mold. According to the blow molding method, the molding material is pinched by the parting surfaces of the split mold and the pinched portion does not stretch. Hence, that portion of the molding material which is adjacent the pinched portion tends to become thinner. This tendency is greater as the molded article has a larger depth or a larger size. When it is desired to mold a depressed article having a hollow double wall structure and a depression, the molding material must be pinched by the entire periphery of the parting surfaces of the split mold. Furthermore, since the peripheral edge portion of the opening of such a molded article adjoins the parting surface of the split mold, this peripheral edge portion has the smallest thickness. Particularly, when the molded article has a deep-drawn configuration, the aforesaid localized variation in thickness of the molding material incident to expansion during the molding further increases, and the depth of a molded article by such a molding method is considerably restricted.
When a male mold member is used in the production of a molded article having a depression, its molding surface pushes the parison at the time of closing the split mold, or the internal air pressure of the tubular parison causes the peripheral portion of the parison to swell out of the mold, whereby that portion of the parison which gets out of the mold sometimes becomes thin. As a result, molding of the product into the desired shape and configuration is impossible or the product has so small a wall thickness that it cannot be used in practical applications.
As an improvement in the molding of a molded article having a depression and a hollow double wall structure from a molding material by a split mold, there is known a method in which a slidable cylindrical member is provided on the periphery of one mold member, the molding material is enclosed in a restricted space surrounded by the cylindrical member and the other mold member while the split mold is kept open, and then the split mold is closed to perform molding. Such a method is disclosed, for example, in Japanese Patent Publication No. 14152/1971 (corresponding to U.S. Pat. No. 3,736,201, British Pat. No. 1,346,499, and West German Pat. No. 2,137,183). Since according to this method, the molding material is not expanded within the closed split mold but is molded within the above-mentioned restricted space, the local thinning of the parison during blow molding can be prevented and a molded article having a depression of considerable depth can be produced. This prior method, however, additionally requires the cylindrical member for forming a restricted space together with one mold member.
Another conventional improved method for molding a hollow article by using a split mold involves using blow molding and vacuum molding simultaneously by providing vacuum suction holes on the surface of the mold cavities of an ordinary blow mold. This method is disclosed, for example, in Japanese Laid-Open Patent Publication No. 112965/1979 filed by the present inventor. It has been found however that according to this method, it is difficult to produce molded articles containing a plurality of closed hollow portions and having a complex configuration.